1. Field of the Invention
The present invention relates to a nickel-metal hydride secondary cell comprising a negative electrode containing hydrogen absorbing alloy particles, a method of manufacturing said cell, hydrogen absorbing alloy particles for cells, and a method of manufacturing said particles.
2. Description of the Related Art
The prominent progress made recently in the electronic technology has achieved a prominent power saving. The recent progress in the mounting technique is also outstanding. As a result, the electronic apparatus has been prominently miniaturized and made portable. Naturally, high demands are being raised for the increase in the capacity of the secondary cell which is incorporated as the power source into the electronic apparatus. Proposed as such a secondary cell is an alkali secondary cell constructed such that hydrogen absorbing alloy particles forming the negative electrode of the cell are fixed to a conductive core acting as a current collector. The alkali secondary cell of this construction has began to attract attentions in this technical field. An AB.sub.5 type alloy represented by LaNi.sub.5 is a typical example of the hydrogen absorbing alloy. The negative electrode using the AB.sub.5 type alloy is advantageous over the negative electrode formed of a typical conventional material of cadmium in terms of the unit weight or energy density per unit volume of the alkali secondary cell. It follows that the use of the AB.sub.5 type alloy makes it possible to increase the cell capacity Also, the use of the AB.sub.5 type alloy is substantially free from the environmental pollution problem. In addition, the alkali secondary cell using the AB.sub.5 type alloy exhibits excellent cell characteristics.
However, the negative electrode containing the hydrogen absorbing alloy particles gives rise to the problem that the volume of the negative electrode is expanded or shrunk when hydrogen is absorbed by or released from the hydrogen absorbing alloy particles during the charging/discharging step of the cell, with the result that the hydrogen absorbing alloy particles are cracked, leading to pulverization of the particles. In accordance with the progress in the pulverization of the hydrogen absorbing alloy particles, the specific surface area of the hydrogen absorbing alloy particles is rapidly increased. Naturally, the surface of the alloy particle is in contact with an alkaline electrolyte and, thus, is deteriorated with time. It follows that the pulverization causes an increased ratio of the deteriorated region to the entire region of the hydrogen absorbing alloy particles. In addition, the electrical conductivity between the hydrogen absorbing alloy particles and the current conductor is deteriorated, leading to deterioration in the electrode characteristics and to reduction in the cycle life.
Various measures are proposed in an attempt to overcome the above-noted difficulty. For example, it is proposed to form a thin nickel film on the surface of the hydrogen absorbing alloy particle by means of plating or vapor deposition so as to increase the mechanical strength of the alloy particle and, thus, to prevent the alloy particle from being cracked. It is also proposed to dip the hydrogen absorbing alloy particles in an alkaline solution, followed by drying the alloy particles so as to suppress the deterioration in the surface of the hydrogen absorbing alloy particle. However, any of these conventional measures fails to achieve a sufficient improvement of the difficulty.
On the other hand, a pulverizing step is required for preparation of hydrogen absorbing alloy particles of uniform particle size in the manufacture of a negative electrode containing the hydrogen absorbing alloy particles. The conventional methods of pulverizing the hydrogen absorbing alloy include, for example, a method in which hydrogen is absorbed by and released from an ingot of the hydrogen absorbing alloy so as to pulverize the ingot and a mechanical pulverizing method using, for example, a ball mill or a cutter mill.
However, the method in which hydrogen is absorbed by and released from an ingot of the hydrogen absorbing alloy is defective in that the hydrogen absorbing alloy particles tend to be ignited if the hydrogen withdrawal after the pulverization is incomplete In addition, only a slight nonuniformity of the alloy composition causes the pulverized alloy particles to be different from each other in the particle size, leading to a low yield in the subsequent step of classification of the pulverized particles. The mechanical pulverizing method is also defective in that the particle size distribution of the pulverized particles tends to be very broad, making it necessary to employ classification after the pulverizing step. The classifying operation leads to a marked reduction in the yield. In addition, fine particles having a particle diameter of 1 .mu.m or less are likely to be formed in the case of employing the mechanical pulverizing method. Naturally, the cell characteristics are adversely affected when the hydrogen absorbing alloy particles prepared by the mechanical pulverizing method are used for preparing the negative electrode. What should also be noted is that a cast ingot is used as the starting material in any of the conventional pulverizing methods described above. Thus, it is necessary to employ a heat treatment for homogenization in order to suppress the segregation in the casting step. Also, it is very difficult to eliminate the micro segregation in spite of the heat treatment for homogenization. The micro segregation causes the formation of a local cell and, thus, the corrosion proceeds. Also, the segregation at the grain boundary causes reduction in the mechanical strength of the alloy particles so as to bring about pulverization of the alloy particles. Such being the situation, the cell characteristics are deteriorated with time when it comes to the conventional secondary cell utilizing the hydrogen absorbing alloy particles.